Here are 2 relnotes for libvirt:
New networking scripts [hbrock] 253066
Proposed text:
-----------------
This release of Red Hat Virtualization includes virtual networking as part of
libvirt, the virtualization library. Libvirt has a set of commands that will set
up a virtual NAT/router and private network for all the local guests on a
machine, especially handy for guests that don't need to be routable from
outside, and for virtualization developers on laptops. See the libvirt man page
and documentation at http://libvirt.org for more details. Note that the virtual
networking capability adds a dependency on dnsmasq, which handles dhcp for the
virtual network.
Inactive domains in libvirt [hbrock] 253066
Proposed text:
----------------
This release of Red Hat Virtualization adds to libvirt the ability to manage
inactive virtual machines. This includes adding several commands (virsh define;
virsh undefine) for defining and undefining domains separately from starting and
stopping them. From a convenience standpoint, it means that the Red Hat Virtual
Machine Manager now displays all available guests and allows you to start them
from the gui.

adding to RHEL5.1 release notes updates:
<quote>
* Virtual networking is now part of libvirt, the virtualization library. libvirt
has a set of commands that sets up a virtual NAT/router and private network for
all local guests on a machine. This is especially useful for guests that do not
need to be routable from the outside. It is also useful for developers who use
Virtualization on laptops.
Note that the virtual networking capability adds a dependency on dnsmasq, which
handles dhcp for the virtual network.
For more information about libvirt, refer to http://libvirt.org.
* libvirt can now manage inactive virtual machines. libvirt is able to do so by
defining and undefining domains without stopping or starting them. This
functionality is similar to the virsh define and virsh undefine commands.
This enhancement allows the Red Hat Virtual Machine Manager to display all
available guests. This allows you to start these guests directly from the GUI.
</quote>